Working Electrodes
Electrode Polishing
Electrochemical Pretreatment
Electrode Polishing
The fundamental process in electrochemical reactions is the transfer of electrons between the working electrode surface and molecules in the interfacial region (either in solution or immobilized at the electrode surface). The kinetics of this heterogeneous process can be significantly affected by the microstructure and roughness of the electrode surface, the blocking of active sites on the electrode surface by adsorbed materials, and the nature of the functional groups (e.g., oxides) present on the surface. Therefore, there has been considerable effort devoted to finding methods that remove adsorbed species from the electrode and produce an electrode surface that generates reproducible results.
The most common method for surface preparation is mechanical polishing. The protocol used for polishing depends on the application for which the electrode is being used, and the state of the electrode surface. There are a variety of different materials available (e.g., diamond, alumina, silicon carbide), with different particle sizes suspended in solution BASi supplies 0.05 mm alumina polish, and 1, 3, 6, and 15 mm diamond polishes; these should be shaken well before use to ensure that the particles are suspended). The pad used for polishing also depends on the material being used for polishing - Texmet pads are used with alumina polish, and nylon pads should be used with diamond polish. Working electrodes supplied by BASi have first been lapped to produce a flat surface, and have then been extensively polished to a smooth, mirror-like finish at the factory. Therefore, they typically only require repolishing with 0.05 mm or 1 mm diamond polish by the user in between experiments. The electrode should be moved in a figure-of-eight motion when polishing to ensure uniform polishing. Materials that have a rougher surface (e.g., commercial electrodes that have been scratched) must first be polished using a larger particle polish, in order to remove the surface defects. After the defects have been removed, the polishing should continue with successively smaller particle size polish (e.g., 15 mm, then 6 mm, then 3 mm, and then 1 mm).
Once polishing has been completed (this can require from 30 s to several minutes, depending upon the state of the electrode), the electrode surface must be rinsed thoroughly with an appropriate solvent to remove all traces of the polishing material (since its presence can affect the electron transfer kinetics). Alumina polishes should be rinsed with distilled water, and diamond polishes with methanol or ethanol. The rinsing solution should be sprayed directly onto the electrode surface. After the surface has been rinsed, electrodes polished with alumina should also be sonicated in distilled water for a few minutes to ensure complete removal of the alumina particles. If more than one type of polish is used, then the electrode surface should be thoroughly rinsed between the different polishes.
As discussed above, the effect of any surface pretreatment can be determined by its effect on the rate of electron transfer. This can be judged qualitatively by examining the separation of the peak potentials in a cyclic voltammogram of a molecule whose electron transfer kinetics are known to be sensitive to the state of the surface (a more quantitative determination can be made by calculating the value of the standard heterogeneous rate constant ks from this peak potential separation). For example, ks for potassium ferricyanide at a glassy carbon surface following a simple polishing protocol is typically in the range 0.01 - 0.001 cm s-1 (this should be compared with the values measured for ks for a platinum electrode, which are at least one order of magnitude larger). The strong dependence of the electron transfer kinetics of ferricyanide on the state of the electrode surface means that there can be significant variations in the peak potential separation after each polishing, since polishing alters the microstructure, roughness, and functional groups of the electrode surface in addition to removing adsorbed species. The materials used for the polishing can also affect the value of ks. For example, the electrode surface can be contaminated by the agglomerating agents required to keep the alumina particles suspended in solution and by the components of the polishing pad. The presence of these species can have a deleterious effect on the electron transfer kinetics by blocking the active sites for the electron transfer reaction. However, it should be noted that such pronounced dependence on the state of the electrode surface is only observed for certain systems (the most well characterized examples are the reduction of ferricyanide, the oxidation of ascorbate, and the adsorption of dopamine). For such systems, polishing is often used in combination with another pretreatment (e.g., heat or electrochemical). However, for many other systems, the simple polishing described above is adequate (for example, when using non-aqueous electrolytes, since blocking of active sites by adsorbed species is less common in such electrolytes than it is in aqueous electrolytes).
Electrochemical Pretreatment
Another method for preparation of the electrode surface that is becoming more widely used is electrochemical pretreatment (ECP), particularly for electrodes which cannot readily be polished (e.g., carbon fiber cylinder electrodes). ECP consists of applying conditioning potentials to the electrode surface before the experiment. As with polishing, this has the effect of removing adsorbed species and altering the microstructure, roughness and functional groups of the electrode surface. The precise ECP protocol depends upon the application, and varies considerably. The potential waveforms typically are either held at, or cycle to, a large positive or negative potential, either using steps or sweeps (constant potential, potential scan, triangular wave, and square wave). Although the development of the preconditioning protocols has been largely empirical, there has been some characterization of the pretreated electrode surface in order to elucidate the reasons for the activation of the electrode surface. For glassy carbon electrode, in addition to the removal of adsorbed species, the preconditioning potential leads to the formation of an oxygen-rich layer on the carbon surface. This layer contains oxides as well as other oxygen-containing functional groups which may catalyze electron transfer reactions (the composition of the functional groups in this layer is sensitive to the pretreatment conditions, and depends on the solution pH as well as the potentials used for the pretreatment). The oxide layer can also adsorb and/or exchange ions from the solution, which leads to improved detection limits. However, electrochemical pretreatment of electrodes can increase the background current of the electrode relative to that of a polished electrode, which may be disadvantageous for some applications.
